1. Field of the Invention
This invention relates to fluorine-doped tin oxide coatings, and more particularly, to an improved liquid coating composition for producing high quality fluorine-doped tin oxide coatings having a low sheet resistance and high visible light transmission.
2. Description of the Prior Art
Fluorine-doped tin oxide coatings are known to impart useful properties to many different substrate surfaces, including glass, ceramics, metals and elemental filaments. Such coated substrates find use as heat reflective elements, in energy efficient windows and in opto-electronic and semiconductor devices.
Several methods have been described in the literature to make such tin oxide coatings, including solution spray, chemical vapor deposition, powder deposition and ion sputtering. See, for example, U.S. Pat. Nos. 3,677,814; 3,759,743; 3,949,146; 4,130,673; 4,265,974; 4,293,594; 4,325,988; 4,389,238; 4,500,567; German Pat. No. 3,010,077 (1980); Japan Kokai No. 75 61 415 (1975); Japan Kokai No. 75 61 416 (1975); Japan Kokai No. 75 61695 (1975); and U.S.S.R. No. 142,000 (Chem. Abs. 56, 13676f).
These methods all suffer from one or more disadvantages in that the process conditions, or compositions used, or the properties of the resulting tin oxide coatings, are not entirely satisfactory for certain commercial applications. Where the product is an energy efficient window, for example, success has been limited by the relatively high sheet resistance and low visible transparency of the coatings, or by the cost of the process.
The solution spray method is described, for example, in Japan Kokai No. 75 61,415. In this method a solution of dibutyltin diacetate and ethyl trifluoroacetate in isopropyl alcohol as a solvent is sprayed onto a glass plate to provide fluorine-doped tin oxide coatings. However, a large amount of solvent is present in the solution, generally about 50 wt %, which must be removed during the process. Furthermore, the electrical resistivity of the coatings obtained have an unacceptable value of 230 ohms/square.
A chemical vapor deposition method is disclosed by Kato et al. in U.S. Pat. No. 4,500,567. The process uses a gaseous mixture of butyltin trichloride and dichlorodifluoromethane which is formed from separate gaseous streams of each reactant. The gaseous mixture then is deposited onto a glass surface. However, in this method, it is inconvenient to control the flow rates of the gaseous streams, and the electrical properties of the coating are not adequate.
Gordon, in U.S. Pat. No. 4,265,974, teaches that mixtures of tetramethyltin and .alpha.-fluoroalkyl halides can be decomposed to provide fluorine-doped tin oxide films with good optical and electrical properties. However, the reaction mixture is explosive in air above 1.9 wt % tetramethyltin, which severely limits the rate of deposition of the coating on sheet glass. In addition, tetramethyltin and its decomposition by-products are very toxic.
Yoldas, in U.S. Pat. No. 4,293,594, discloses a vapor deposition method of forming a highly conductive transparent fluorine-doped tin oxide coating for fluorescent lamps. The patentee uses a gaseous mixture of dimethyltin dichloride and dimethyltin difluoride in an oxygen-containing carrier gas to produce the coating. However, these tin compounds are solids which must be vaporized at high temperatures to provide a gaseous mixture.
As will be apparent from the above-described review of the prior art, there is a need for an improved process of making high quality, high performance fluorine-doped tin oxide coatings.
Accordingly, it is an object of this invention to provide an improved method for forming high quality, high performance fluorine-doped tin oxide coatings.
A particular object herein is to provide an improved liquid coating composition for the production of such fluorine-doped tin oxide coatings under advantageous process conditions.
Still another object herein is to provide a liquid coating composition which includes the reaction product of a fluorine dopant and organotin compounds in predetermined concentration ranges, from which fluorine-doped tin oxide coatings which have a low sheet resistance and high visible transparency may be made in a relatively short deposition time.